The present invention relates, generally, to an exposure apparatus, and, more particularly, to an immersion type exposure apparatus that fills a space between a final surface in a projection optical system and a surface in a plate to be exposed with a liquid, and exposes the plate via the projection optical system and the liquid.
A projection exposure apparatus has conventionally been used to transfer a circuit pattern on a reticle (or a mask) via a projection optical system onto a wafer, etc., and high-quality exposure at a high resolution has recently been increasingly demanded. The immersion type exposure has attracted attention as one means that satisfies this demand. The immersion type exposure promotes a higher numerical aperture (“NA”) of the projection optical system by replacing a medium (typically, air) at the wafer side of the projection optical system with a liquid. The projection optical system has an NA=n·sin θ, where n is a refractive index of the medium, and the NA increases when the medium that has a refractive index higher than the refractive index of air, i.e., n>1. As a result, the resolution R (R=k1(λ/NA)) of the exposure apparatus defined by a process constant k1 and a light source wavelength λ becomes small.
However, if the refractive index of the liquid (immersion liquid) is changed with time by a change of a temperature or component, an aberration, such as a focal error, a spherical aberration, and a curvature of field, occurs and an imaging performance of the projection optical system deteriorates. The exposure apparatus needs to precisely transfer a reticle pattern onto a wafer, and due to the recent demands for finer processing to the semiconductor devices, the transferred pattern becomes more sensitive to the aberration of the optical system. In particular, due to further higher NA's of the projection optical system, an aqueous solution and an organic material investigated as an adoption to the liquid have provided a large change of the refractive index.
Then, an immersion type exposure apparatus that includes an adjusting part for adjusting the refractive index of the liquid has been proposed. See, for example, Japanese Patent Applications, Publication Nos. 10-340846 and 2004-301825.
However, Japanese Patent Application, Publication No. 10-340846, measures the aberration of the projection optical system, calculates a change amount of the refractive index based on the result and adjusts the refractive index. The measurement of the aberration needs a long time. Then, when the refractive index is changed, Japanese Patent Application, Publication No. 10-340846, cannot quickly adjust this. Therefore, it is not preferable to use the aberration measuring apparatus of the projection optical system to correct the change of the refractive index of the liquid. In addition, a correspondence with the aberration and the refractive index is not clear. Japanese Patent Application, Publication No. 2004-301825, projects plural detected light beams onto a substrate with a different degree of an incident angle and obtains a change amount of the refractive index based on a difference of an error amount at a surface position. Therefore, if the refractive index of the liquid is changed by a factor of both the temperature and the component, a refractive index with a high precision is not obtained.